Issue links in description/comments are not linkified

[Tyriar]

See the link at the bottom. repo/user#num format should also be covered

[rebornix]

related #74

Right now the description we get from GitHub v3 API is plain text (markdown) so we can render the content to HTML but we didn't do any conversion like converting #1518 to https://github.com/${owner}/${repo}/issues/1518, or truncating the label for links.

As we are using Markdown-it, we can customize the markdown rendering to support this or maybe GitHub API can provide content similar to what they already have in web pages.

[Tyriar]

@rebornix couldn't you run the markdown through a filter which converts the links before passing it to markdown-it?

[rebornix]

@Tyriar markdown grammars are ambiguous so writing a markdown-it extension which takes care link token can solve this problem better.

[rebornix]

As raised in #105, links in comments are not linkified as well.

[RMacfarlane]

Completed by #831